Range measurement device

ABSTRACT

A range measurement device is disclosed. The device comprises a flash laser radar configured to produce a first laser pulse at a first time. The device receives, at a second time, reflections of the first laser pulse from at least one object within a 360 degree field of view. The device further comprises a timing electronics module, an image sensor in communication with the timing electronics module, a mirror element coupled between the image sensor and the laser radar, and a lens. The mirror element includes a first reflector configured to disperse the reflections of the first laser pulse within at least a portion of the 360 degree field of view and a second reflector configured to collect returning reflections of the first laser pulse from the at least one object into the image sensor. The lens is configured to focus the returning reflections onto the image sensor.

This application is related to commonly assigned and co-pending U.S.patent application Ser. No. 11/678,313 filed on Feb. 23, 2007 andentitled “CORRELATION POSITION DETERMINATION” (the '313 Application).The '313 Application is incorporated herein by reference.

BACKGROUND

Many navigation applications provide precise locating and tracking ofobjects when necessary. For example, unmanned vehicles, such as anunmanned ground vehicle (UGV), require accurate position information inorder to properly navigate an area. Most of these navigationapplications employ one or more global positioning system (GPS) sensorsto achieve a necessary level of precision.

The use of GPS, however, has some limitations. For example, the GPSsignals may not be available in a desired location where satellitecommunication is blocked or scrambled. In addition, the GPS sensors donot obtain any local features of the area (for example, any additionalsurrounding objects or landmarks within the area) relative to thelocation of the vehicle. To date, tracking any localized featuresrequires additional synchronization time in instances where accuratemeasurements within a minimal time period are critical.

For the reasons stated above and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art forimprovements in range measurements and object position tracking.

SUMMARY

The following specification discusses a range measurement device. Thissummary is made by way of example and not by way of limitation. It ismerely provided to aid the reader in understanding some aspects of atleast one embodiment described in the following specification.

Particularly, in one embodiment, a range measurement device is provided.The device comprises a flash laser radar configured to produce a firstlaser pulse at a first time. The device receives, at a second time,reflections of the first laser pulse from at least one object within a360 degree field of view. The device further comprises a timingelectronics module, an image sensor in communication with the timingelectronics module, a mirror element coupled between the image sensorand the laser radar, and a lens. The mirror element includes a firstreflector configured to disperse the reflections of the first laserpulse within at least a portion of the 360 degree field of view and asecond reflector configured to collect returning reflections of thefirst laser pulse from the at least one object into the image sensor.The lens is configured to focus the returning reflections onto the imagesensor.

DRAWINGS

These and other features, aspects, and advantages are better understoodwith regard to the following description, appended claims, andaccompanying drawings where:

FIG. 1 is a block diagram of a navigation system;

FIG. 2 is a traverse diagram of a vehicle having the system of FIG. 1traversing through an area;

FIG. 3 is a block diagram of a range measurement device;

FIG. 4 is a flow diagram of a method for measuring range; and

FIG. 5 is a flow diagram of an alternate method for measuring range.

The various described features are drawn to emphasize features relevantto the teachings of the present application. Like reference charactersdenote like elements throughout the figures and text of thespecification.

DETAILED DESCRIPTION

Embodiments of the present invention relate to a range measurementdevice that determines an absolute location and attitude of the systemwith respect to local objects with absolute known locations (referred toherein as targeted objects) and, in at least one alternate application,a relative location and attitude of the system with respect to any localobjects within range of the range measurement device (referred to hereinas surrounding objects). In one embodiment, the range measurement devicediscussed herein determines the location of the system using a pulsefrom a laser radar (LADAR). Moreover, the range measurement deviceprovides synchronized measurements of the surrounding objects within a360° field of view along the horizon and returns the distance andangular relationship between the range measurement device and thesurrounding objects. With these distance and angular relationships, alocation and attitude of the system is determined within a localizedarea with respect to the surrounding objects. In one implementation, therange measurement device mounts on an unmanned ground vehicle (UGV)interested in at least one of recording an absolute location of the UGV,tracking another vehicle or object within the localized area, anddetermining a relative location and attitude of the UGV within thelocalized area.

The use of a LADAR improves range tracking accuracy. The rangemeasurement device discussed here measures an accurate range as well asthe angular relationship of the surrounding objects relative to therange measurement device. In one implementation, range, location, andazimuth measurements of the surrounding objects are synchronouslymeasured to a plurality of three-dimensional (3-D) images using theLADAR output. The accurate range measurements are used to locate thedevice either absolutely or relatively (which is useful in 3-D renderingand mapping applications and allows for localization and mapping in asingle process).

In one implementation, the LADAR produces a laser pulse and acharge-coupled device (CCD) sensitive to the laser pulse receives atleast one reflection from the surrounding objects “of interest.” Basedon the reflection(s) received, the CCD can measure a flight time of thelaser pulse. In the same and at least one alternate implementation,measurements from the CCD are used by a processor in the rangemeasurement device to determine the location of the UGV relative to anyof the surrounding objects identified. Moreover, given the absolutelocation of the UGV, location coordinates (for example, latitude,longitude, and altitude) of the object are produced. Likewise, the 3-Dinformation can be used to determine the attitude of the object (forexample, heading pitch and roll).

FIG. 1 is a block diagram of a navigation system 100. As illustrated,the navigation system 100 includes a processor 102 that is incommunication with a range measurement device 106 (which, in thisembodiment, comprises a 360° 3-D flash LADAR). In some embodiments, thesystem 100 includes at least one additional sensor such as a GPS sensor108, an inertial sensor 110, a heading sensor 112, a speed sensor 114,and an altimeter sensor 116.

The range measurement device 106 provides range data, includingdistances and angles, to the processor 102 of objects near a hostvehicle having the system 100. Many methods of communication arepossible including CCD pixel information, however, one skilled in theart can determine methods for providing distance and angle information.As indicated above, in one embodiment, a 360° 3-D flash LADAR is used inthe range measurement device 106. The LADAR-based range measurementdevice 106 detects and locates the objects using a single flash of laserlight and provides information similar to radar. Moreover, the rangemeasurement device 106 illuminates a 360° field of view so that objectsin any direction along the horizon from the range measurement device 106are located (for example, located as the host vehicle traversesthroughout an area, similar to the area discussed below with respect toFIG. 2). As the host vehicle passes through the area, the rangemeasurement device 106 tracks individual angles and ranges between theobjects and the range measurement device 106 (for example, relativedistance data). When absolute position information for the objects isknown, the relative distance data can be transformed to Earth-referencedangles for the objects in a specified area (for example, relativedistance data). Both the relative and absolute distance data areprocessed in the processor 102.

In the embodiment that includes the inertial sensor 110, additionalinformation is provided to the processor 102 to estimate the location ofthe host vehicle. Generally, the inertial sensor 110 estimates a presentposition based on a prior knowledge of time, initial position, initialvelocity, initial orientation without the aid of external information.As illustrated in FIG. 1, an initial position input and an initialheading input is provided. The information generated by the inertialsensor 110 (in this embodiment) is provided to the processor 102. Theprocessor 102 uses the inertial sensor 110 data, in combination with thedistance and angle data, from the ranging device 106 to determine thecurrent location of the host vehicle. The current location and currentheading is output as illustrated in FIG. 1. The output of the currentheading and current location is used to position the system 100 withboth absolute and relative navigation coordinates.

FIG. 2 is a traverse diagram 200 illustrating a host vehicle 207 (forexample, a UGV) passing through an area 200 to be traversed. Asillustrated, the area 200 to be traversed includes objects 210 ₁ through210 _(N). In one implementation, the objects 210 ₁ through 210 _(N) aremapped (for example, previously located) according to a correlationposition determination method as disclosed in the '313 Application. Inalternate implementations, locations of the objects 210 ₁ through 210_(N) are not previously known. The host vehicle 207 takes a path 206that starts at a first point 202 and ends at a second point 204. Thehost vehicle 207 comprises a navigation system 208 including the rangemeasurement device 106 of FIG. 1. In the example embodiment of FIG. 2,the range measurement device 106 of the navigation system 208 transmits(for example, flashes a laser light pulse from a flash LADAR) andreceives a 3-D image of the objects 210 through the use of a mirrorelement and an image sensor, as further described below with respect toFIG. 3.

FIG. 3 is a block diagram of a range measurement device 300. In theexample embodiment of FIG. 3, the device 300 represents the rangemeasurement device 106 of FIG. 1. The device 300 in this embodimentcomprises a flash laser radar (LADAR) 302, a mirror element 304(comprising a first reflector 314 and a second reflector 316), and atiming electronics module 306 in communication with the image sensor 310and a lens 308. In the example embodiment of FIG. 3, the timingelectronics module 306 monitors sensitivity of the image sensor 310 bycontrolling a shutter of the lens 308 with a “Shutter Control Signal”communication link. Moreover, the timing electronics module 306 controlsthe initiation of a first laser pulse originating from the flash LADAR302 to illuminate a target (for example, the objects 210 of FIG. 2) witha “Flash Control Signal” communication link. In one implementation, thetiming electronics modules 306 further comprises one or more timers 312operable to measure signal transit timing of the first laser pulse witha “Timing Control Signal” communication link as shown in FIG. 3.

In the example embodiment of FIG. 3, the image sensor 310 is acharge-coupled device (CCD). The image sensor 310 consists of anintegrated circuit containing an array of linked, or coupled,light-sensitive capacitors (that is, pixels). In one implementation, themirror element 304 is a 3-D reflective surface. Moreover, the firstreflector 314 is one of a cone, a trapezoid, a sphere, or any othersurface which can disperse the first laser pulse into the requiredfield, and the second reflector 316 is one of a cone, a trapezoid, asphere, or any other surface which can focus a first reflected laserpulse into the lens 308, as further discussed below. The image sensor310 contains at least one pixel grid array to sense the returned 3-Dreflections from the mirror element 304. As discussed in further detailbelow, the image sensor 310 further provides the intensity of thereflection to the processor 102 and terminates the signal transit timingin the timing electronics module 306.

In operation, the flash LADAR 302 illuminates the objects 210surrounding the device 300 in a 360° radius through a reflective surface(for example, the mirror element 304). In one implementation, the firstreflector 314 is configured to disperse reflections of a first laserpulse into a 360° field of view. The second reflector 316 collectsreturning laser pulse reflections of the first laser pulse onto the lens308. In one or more alternate implementations, at least a portion of thefield of view less than 360° can be selected. For example, the firstlaser pulse from the flash LADAR 302 reflects in all directions (thatis, the first laser pulse reflects 360°) from the first reflector 314and returns the reflections back from the objects 210 to the secondreflector 316. In turn, the lens 308 focuses the return reflections fromthe second reflector 316 on a focal plane of the image sensor 310. Fromthe return reflection(s) of the second reflector 316, the lens 308captures an image of at least one of the objects 210 for the imagesensor 310. In the example embodiment of FIG. 3, the focused reflectioncaptured by the lens 308 causes each capacitor in the pixel array of theimage sensor 310 to accumulate an electric charge proportional to thelight intensity at the location of each of the objects 210.

Using the pixel intensity locations from the image sensor 310, relativeangular measurements of each of the objects 210 are taken by theprocessor 102. A local distance between the flash LADAR 302 and thesurrounding objects 210 is also determined given a propagation time ofthe flash LADAR 302 from flash to reception taken at each pixel of theimage sensor 310. A relative range for each of the objects 210 iscomputed given the local distance and a relative velocity (for example,the speed of light) of the first laser pulse return reflections. Therelative range is calculated based on the length of time it takes forthe light (for example, the first laser pulse) to return from thetarget.

Using the relative angular and range information, a 3-D image iscreated. In one embodiment, once the lens 308 receives the reflectionimage, independent pixels within the image sensor 310 stop the timers312 of the timing electronics module 306. The image sensor 310 measuresthe change in light intensity to stop the signal transit timing(initiated at the time the “Flash Control Signal” is sent from thetiming electronics module 306 to the flash LADAR 302) to determine therelative range to each of the objects 210 for each illuminated pixel inthe image sensor 310. The propagation time between initializing thefirst laser pulse and the received reflection image is proportional tothe range from the range measurement device 300 to the objects 210 basedon the relative velocity of the first laser pulse return reflections. Inone implementation, range, location, and azimuth measurements of theobjects 210 are synchronized to each 3-D image produced.

In one implementation, given the relative angles and distances discussedabove, the host vehicle 207 (of FIG. 2) is localized with as few as twomeasurements. Moreover, the laser pulses available with the flash LADAR302 increase the measurement accuracy of the range measurement device300. In one implementation, any potential objects of interest arelocalized relative to the host vehicle 207 with the range measurementdevice 300 as discussed below.

Localized Navigation Based on LADAR Range Data

In the example embodiments of FIGS. 2 and 3, accurately measured rangesare used to locate the host vehicle 207 to at least four objects 210previously surveyed. In identifying the at least four surveyed objects210 as targeted objects, each pixel on the image sensor 310 recordsrange to whatever illuminated that pixel. In one implementation, onlypixels with very strong returns are considered since those pixels willcorrespond to reflective surveyed markers present on the at least foursurveyed objects 210. It is understood that many means exist foridentifying the targeted objects 210 that are well known in the art. Inone implementation, a first pixel coordinate (i, j) in (for example) a128×128 pixel layer CCD in the image sensor 310 provides an estimate ofa unit-vector towards at least one of the objects 210 that illuminatedthe first pixel at (i, j). Together, the range and unit vector provide afirst estimate of the targeted object 210's location in polarcoordinates. Additionally, this first estimate identifies which of theat least four (surveyed) objects 210 is illuminating the first pixel. Inthe example embodiment of FIG. 2 (given the surveyed ranges of thetargeted objects 210), an accurate estimate of the location of the hostvehicle 207 is determined with Equations 1 to 6 described below, wheresubtracting two range equations (for example, Equations 1 and 2)eliminates the unknown quadratic terms and results in a linear equation(for example, Equation 3).

In one implementation, Equations 1 and 2 illustrated below use Cartesiangrid coordinates (x₁,y₁,z₁) and (x₂,y₂,z₂) for the location of theobjects 210 ₁ and 210 ₂ at two known locations, and a first unknownposition (x₀,y₀,z₀), with measured ranges of r₁ and r₂ from the rangemeasurement device 300 to the objects 210 ₁ and 210 ₂.(x ₀ −x ₁)²+(y ₀ −y ₁)²+(z ₀ −z ₁)² −r ₁ ²=0  Equation 1(x ₀ −x ₂)²+(y ₀ −y ₂)²+(z ₀ −z ₂)² r ₂ ²=0  Equation 2

As illustrated above, the only unknown quadratic terms in Equations 1and 2 are x₀ ²+y₀ ²+z₀ ². By subtracting Equations 1 and 2, all theunknown quadratic terms cancel (for example, Equation 1−Equation 2=0) asillustrated below in Equation 3.2*((x ₂ ,y ₂ ,z ₂)−(x ₁ ,y ₁ ,z ₁)*(x ₀ ,y ₀ ,z ₀)+r ₂ ² −r ₁ ²+∥(x ₁ ,y₁ ,z ₁)∥²−∥(x ₂ ,y ₂ ,z ₂)∥²=0  Equation 3

Equation 3 as discussed above is a linear equation with unknowns(x₀,y₀,z₀). The symbol ∥ ∥² is defined as illustrated below in Equation4.∥(x _(n) ,y _(n) ,z _(n))∥² =x _(n) ² +y _(n) ² +z _(n) ² where n=1,2, .. .  Equation 4

In the example embodiment of FIG. 2, the at least four surveyed markers(that is, the objects 210 ₁ to 210 ₄) result in at least six linearequations from the six combinations (for example, (1−2), (1−3), (1−4),(2−3), (2−4) and (3−4)) as shown below:(Equation 1)−(Equation 2)=0(Equation 1)−(Equation 3)=0(Equation 1)−(Equation 4)=0(Equation 2)−(Equation 3)=0(Equation 2)−(Equation 4)=0(Equation 3)−(Equation 4)=0

The above example provides at least 6 linear equations with at least 3unknowns (x₀,y₀,z₀), which are solved using commonly-known linearalgebra and a least squares solution. These equations are written invector notation as illustrated below in Equation 5, with a least squaressolution to [{circumflex over (x)}] shown below in Equation 6.[A][x]=[b]  Equation 5[{circumflex over (x)}]=[A^(T)A]⁻¹[A^(T)][b]  Equation 6

The result of Equation 6 is a location of the host vehicle 207,[{circumflex over (x)}], localized to the targeted objects 210 using therange data of the flash LADAR 302.

FIG. 4 is a flow diagram of a method 400 for measuring range todetermine a laser radar position. For example, the method 400 uses arange measurement device similar to the range measurement device 300 ofFIG. 3 to perform 360° range tracking of an area surrounding a vehicle(for example, the host vehicle 207 of FIG. 2). The method discussed herereturns the distance and angular relationship between the vehicle andany surrounding objects currently positioned in the area. For example,the range measurement device 300 includes a flash, a mirror element, anda CCD-based image sensor so that the surrounding objects can be detectedin a 360° field of view.

At block 402, the range measurement device produces a first laser pulsewith the laser radar at a first time dispersed over the 360° field ofview. In one implementation, a timing electronics module of the rangemeasurement device controls the initiation of the first laser pulse inorder to properly illuminate at least one surrounding object. The rangemeasurement device receives reflections of the first laser pulsereflected from surrounding objects within the 360 degree field of viewat a second time (block 404). In one implementation, the second timecomprises a relative transit time of the reflected first laser pulsewithin a localized time period between the first and second times. Basedon the reflections received, the range measurement device tracks atleast one of a range and an attitude from the vehicle to the at leastone surrounding object (block 406). In one implementation, signaltransit times between producing the flash pulse to reception at theCCD-based image sensor and pixel intensity locations measured by theCCD-based image sensor are used to determine the relative angles andrelative ranges of the surrounding objects.

As additional reflections are received, the range measurement devicedetermines additional localized time periods to determine at least oneof a current position and a current attitude of the host vehiclerelative to the surrounding objects (block 408). In one implementation,computing performed at block 408 comprises measuring the additionallocalized time periods to determine a current heading, pitch, and rollof the vehicle. In addition, based on the reflections received and aknown absolute position of the targeted objects, the range measurementdevice determines an absolute position of the vehicle, records at leastone relative position angle between the target and the vehicle, andcomputes at least one of an absolute position and absolute attitude tothe targeted objects (block 410).

FIG. 5 is a flow diagram of a method 500 for measuring range todetermine target positions. For example, the method 500 addresses usinga first laser pulse from the range measurement device 300 of FIG. 3 toperform 360° range tracking of an area surrounding a vehicle (forexample, the host vehicle 207 of FIG. 2) and returns the distance andangular relationship between the vehicle and any (known) targetedobjects currently positioned in the area. The range measurement deviceused for the method of FIG. 5 includes a flash, a mirror element, and aCCD-based image sensor for the targeted objects of the objects 210 to bedetected in a 360° field of view.

At block 502, the range measurement device produces a first laser pulsewith the laser radar at a first time dispersed over the 360° field ofview. In one implementation, a timing electronics module of the rangemeasurement device controls the initiation of the first laser pulse inorder to properly illuminate at least one surrounding object. The rangemeasurement device receives reflections of the first laser pulse fromsurrounding objects within the 360 degree field of view at a second time(block 504). In one implementation, the second time comprises a relativetransit time of the reflected first laser pulse within a localized timeperiod between the first and second times. Based on the reflectionsreceived, the range measurement device tracks at least one of a rangeand an attitude from the vehicle to the at least one surrounding object(block 506). In one implementation, signal transit times betweenproducing the flash pulse to reception at the CCD-based image sensor andpixel intensity locations measured by the CCD-based image sensor areused to determine the relative angles and relative ranges of thetargeted objects. As additional reflections are received, the rangemeasurement device determines additional localized time periods todetermine at least one of a relative position and relative attitude tothe targeted objects (block 508). In addition, based on a known absoluteposition of the laser radar, the range measurement device computes atleast one of an absolute position and absolute attitude to the targetedobjects at block 510.

While the embodiments disclosed have been described in the context of anavigational system, apparatus embodying these techniques are capable ofbeing distributed in the form of a machine-readable medium ofinstructions and a variety of program products that apply equallyregardless of the particular type of signal bearing media actually usedto carry out the distribution. Examples of machine-readable mediainclude recordable-type media, such as a portable memory device; a harddisk drive (HDD); a random-access memory (RAM); a read-only memory(ROM); transmission-type media, such as digital and analogcommunications links; and wired (wireless) communications links usingtransmission forms, such as (for example) radio-frequency (RF) and lightwave transmissions. The variety of program products may take the form ofcoded formats that are decoded for actual use in a particularnavigational system incorporating the range measurement device discussedhere by a combination of digital electronic circuitry and software (orfirmware) residing in a programmable processor (for example, aspecial-purpose processor or a general-purpose processor in a computer).At least one embodiment can be implemented by computer-executableinstructions, such as program product modules, using the programmableprocessor. The computer-executable instructions, any associated datastructures, and the program product modules represent examples ofexecuting the teachings of the present application disclosed herein.

This description has been presented for purposes of illustration, and isnot intended to be exhaustive or limited to the embodiments disclosed.Variations and modifications may occur, which fall within the scope ofthe following claims.

1. A range measurement device, comprising: a flash laser radarconfigured to produce a first laser pulse; a timing electronics moduleincluding one or more timers; an image sensor in communication with thetiming electronics module; a mirror element coupled between the imagesensor and the flash laser radar, the mirror element including: a firstreflector configured to disperse reflections of the first laser pulseinto a 360 degree field of view; and a second reflector configured tocollect returning reflections of the first laser pulse from at least oneobject within the 360 degree field of view into the image sensor; and alens configured to focus the returning reflections of the first laserpulse onto the image sensor.
 2. The device of claim 1, furthercomprising a processor coupled to an output of the image sensor, theprocessor operable to provide range, azimuth, and location measurementswith three-dimensional images produced by the image sensor, anddetermine at least one of a relative location and an absolute locationof a host vehicle having the range measurement device based on therange, azimuth, and location measurements provided.
 3. The device ofclaim 1, wherein the timing electronics module is operable to: monitor asensitivity of the image sensor; and control initiation of the firstlaser pulse to illuminate the target.
 4. The device of claim 3, whereinthe timing electronics module is further operable to measure signaltransit timing of the first laser pulse as a propagation time betweenthe initiation of the first laser pulse and the returning reflections.5. The device of claim 1, wherein the image sensor is a charge-coupleddevice operable to measure a light intensity of the returningreflections.
 6. The device of claim 1, where the first and secondreflectors of the mirror element comprise a single reflector.
 7. Thedevice of claim 1, wherein the second reflector of the mirror elementreturns at least one three-dimensional image of a targeted object.